This invention relates to the fabrication of transistors, and in particular to the fabrication of high speed bipolar transistors.
It is well known that the response time of a bipolar transistor is directly related to the base width and the total base charge. In normally processed bipolar transistors the impurity concentration of the base near the base-collector junction is either gaussian, exponential, or error-function complement. The impurity concentration of the emitter is likewise describable mathematically. To a large extent the base width and the total base charge are inversely proportional to each other. In order to make the base width narrow, the total base doping must be decreased.
A narrow base width and a relatively high total base doping is desirable to produce transistors which have a good high frequency response and therefore high switching speeds. These conditions are difficult to achieve because of the inverse relationship between base width and total base doping.
It would be desirable to be able to fabricate a bipolar transistor having a relatively narrow base width and a relatively large amount of total base charge. This would result in a high speed transistor which has application in analog and digital circuits.